The effect of ultraviolet (UV) irradiation on nanofiltration (NF) membrane biofouling has been studied in pilot scale installations using two identical parallel membrane pilots and a low pressure UV reactor. The two pilots were fed either granular activated carbon (GAC)-filtered water or UV-irradiated GAC-filtered water during a filtration run of 20 weeks. UV pre-treatment did not affect the organic carbon concentration but decreased the active planktonic bacteria counts and increased the dead planktonic bacteria counts of the feed water. Membrane permeability and longitudinal pressure drop (LPD) were continuously monitored during the experiments. UV irradiation was associated with much lower LPD increase, and moderate permeate flux decline. At the end of the test, spiral-wound modules were autopsied and analysed. Membranes of UV-irradiated pilot harboured decreased amounts of biofoulants (global quantity of deposit, sessile bacteria and ATP concentration and amount of extracellular polymeric substances). In conclusion, both membrane performances monitoring and foulant analysis showed that UV irradiation was an efficient NF pre-treatment to reduce NF membrane biofouling. (C) 2011 Elsevier B.V. All rights reserved.